It is only in relatively recent years that society has appreciated the impact and consequences of the fact that we live in a closed ecological system. With an increase in human population and, perhaps more importantly, an increase in industrial activity the effects of ecological changes have become more apparent. One area which has received a great deal of attention is that of water quality, which may be the result of the belated recognition that not only is water of a suitable quality for human consumption a limited resource, but that good water quality is an important, if not critical, factor in the ecological chain. Consequently attention has turned not only to purification of water in local water supplies, but also to limiting the discharge of materials into streams and acquifers generally.
The classes of noxious materials (pollutants) in aqueous discharges vary over an enormously broad spectrum. Among the inorganic pollutants those toxic to a broad spectrum of biological species are especially dangerous. Although heavy metals such as lead, cadmium, and arsenic often are the first culprits thought of, inorganic water soluble cyanide is in a comparably dangerous class because of the generally low tolerance of life forms to cyanide.
The sources of cyanide are many and varied and include iron and steel manufacturing, petroleum and coal pyrolysis processes, the photographic, chemicals, and pharmaceutical industries, precious metal mining and metal finishing, including electroplating and galvanizing. For example, cyanide arises in iron and steel manufacture by reduction of carbonate in the presence of carbon and nitrogen. In power plants coal burning may afford coke oven gas with a hydrogen cyanide concentration on the order of 2 grams per liter. Cyanide solutions are an important component of electroplating and galvanizing, and wash water streams resulting from post-coating treatment often contain significant quantities of cyanide. The widespread prevalence of cyanide in industrial effluents coupled with their near universal toxicity to life has made it imperative to minimize cyanide concentration in aqueous streams.
It appears that the most prevalent methods of cyanide removal are based on the oxidation of cyanide. See generally R. Gierzatowicz et al., Effluent and Water Treatment Journal, 25, 26-31 (1986). Oxidation with chlorine or hypochlorite seems to be industrially the most commonly employed method. The first stage in this oxidation is the formation of cyanogen chloride, CICN, itself a rather toxic gas, but which is hydrolyzed at a high pH to the less toxic cyanate, CNO. Cyanate is itself hydrolyzed to carbon dioxide and ammonia at low pH, or is further oxidized to carbon dioxide and nitrogen. Another oxidative method uses peroxides such as hydrogen peroxide, Caro's acid, peracetic acid, and so on, as the oxidizing agent. The advantages of this approach vis a vis the chlorine or hypochlorite based process is the lack of toxic byproducts and the formation of environmentally neutral species from the peroxides. A disadvantage is the long reaction times necessary for adequate oxidation. However, cupric ions supposedly act as catalysts for peroxide oxidation. Other oxidizing agents based on Mn(VII) and Cr(VI) also have been used.
More recently there has been described the oxidation of both free and complex cyanide in aqueous streams by a mixture of sulfur dioxide or alkali/alkaline earth metal sulfites (including bisulfites and metabisulfites) and air or water in the presence of a water-soluble copper(II) catalyst at a pH between 5 and 12; U.S. Pat. No. 4,537,686. [Although copper is designated as "Cu.sup.+ " in the issued patent, the fact that most cuprous salts are water insoluble and that Cu(I) is readily oxidized strongly suggests that Cu(II) actually was used.] Using rather high weight ratios of copper to cyanide on the order of about 0.25, final cyanide concentrations could be reduced to under 0.1 parts per million. More recently Chen et al. (Paper 81c presented at the 1990 AlChE Summer National Meeting, San Diego, Calif., Aug. 21, 1990) presented data on the oxidation of aqueous streams containing cyanide at 100 ppm using a soluble copper catalyst in conjunction with sodium sulfite and air over activated carbon in a trickle bed reactor at normal pressure. Initially the copper/cyanide molar ratio was about 0.25, but since copper(II) hydroxide precipitated on the carbon surface, it was found that a copper/cyanide maintenance ratio of about 0.1 was quite adequate. Although the authors characterize the activated carbon as a catalyst, this conclusion is far from clear according to the data. Thus, although the authors showed that use of a bed of activated carbon leads to 99% removal of cyanide, beds of both a molecular sieve and glass beads were almost as effective in affording about 80% removal. The improved result with activated carbon could readily be attributed to the extent of copper(II) deposition on the packed bed and its dispersion on the bed materials.
A continuous method for the removal of cyanide using air or oxygen as the oxidizing agent at ambient temperatures and pressures is highly desirable. Although the foregoing references provide a start, much remains before a commercially viable system is operative. In particular, it is often desirable that the catalyst either be heterogeneous, or if homogeneous readily separable, in order to avoid contamination of the effluent by the catalyst itself as well as to minimize process cost associated with catalyst consumption. It also is desirable that the catalyst be relatively insensitive to as large a class of contaminants likely to accompany cyanide as is possible. The process should be capable of efficient operation at atmospheric pressure and preferably as close to ambient temperature as possible in order to minimize energy requirements. Finally, it is desirable for such a process to oxidize the cyanide over a rather wide range of initial cyanide concentrations, and to have the capability of oxidizing 90% or more of the cyanide present.
What we have observed is that a rather broad class of metal chelates are quite effective as catalysts in oxidizing cyanide using only air as the oxidizing agent. These metal chelates can be used either in a soluble form or water-insoluble form so as to afford the opportunity of either homogeneous or heterogeneous catalytic oxidative removal of cyanide, depending upon the needs and/or preferences of the user. We furthermore have found that the oxidation of cyanide as catalyzed by the metal chelates of our invention lead to the formation of carbon dioxide and nitrogen as well as that of isocyanate. Processes which are based on the use of the metal chelates as catalysts for the oxidation of cyanide are effective over rather large initial concentration ranges of cyanide can be readily tailored to particular effluent streams, and consequently are quite versatile.